The present invention relates generally to amplifier circuits and, in particular, an output stage of an amplifier which converts a differential signal to a single-ended signal and which has an output voltage swing that is capable of approaching the upper and lower power supply voltages.
There is an increasing demand for amplifier circuits having the capability of accepting input signals that come very close to the upper and lower supply voltages and capable of providing outputs which have the same capability. This is especially true in those application where the power supply voltages are small. It is also desirable that the amplifier circuit be capable of converting differential input signals to a single-ended output.
FIG. 1 is a schematic diagram of an exemplary amplifier output stage which is preceded by an input stage, not depicted, which converts a differential input into a single-ended current output. Additional details regarding this type of amplifier output stage are set forth in U.S. Pat. No. 4,570,128 entitled xe2x80x9cClass AB Output Circuit With Large Swing.xe2x80x9d In order to permit operation with the differential input near the power supply rails, the input stage is configured to produce two outputs, only one of which is active at any particular time. Thus, the output stage of FIG. 1 is capable of being driven by either In1 or In2.
When the input signal falls within the upper region of the supply voltage range defined by VCC and VEE, signal In1 is active and signal In2 is inactive. Thus, signal In1 will vary with the input signal and signal In2 will be zero. Similarly, when the amplifier input signal is the lower region of the power supply range, signal In2 is active and signal In1 is inactive.
Output transistor QA of the prior art output stage is a common-emitter configured PNP transistor having a collector connected to the amplifier stage output. Similarly, output transistor QB is a common-emitter configured NPN transistor having a collector connected to the amplifier stage output. Output transistor QA is driven by NPN transistor QD. The amplifier output stage of FIG. 1 is configured to operate as a class AB amplifier having a relatively small quiescent current when not driving a load and capable of sourcing and sinking relatively large amounts of current when demanded by a load. The quiescent current flow through output transistors QA and QB from supply VCC to supply VEE is determined by the magnitudes of the bias voltages VA and VB and current sources IA and IB. The output stage is used in a closed loop configuration so that the input In1 or In2 will be adjusted until the output transistors QA and QB conduct equal amounts of current.
When current input In1 is the active input, an increase in current In1 will turn on output transistor QA harder. Current IC through transistor QC will drop since less current is available form source IA. The lower current IC of QC will mean that the current ID of transistor QD must increase since source IB will remain constant. The increase in ID will increase the base current drive of transistor QA thereby reinforcing the effect of an increasing the value of In1. In addition, the increase in current IC will increase the base voltage of output transistor QB thereby causing transistor QB to conduct less current. Thus, voltage Vout will increase towards supply VCC.
Vout will increase towards supply VCC and can be driven until transistor QA is in saturation. In that case Vout is at supply voltage VCC less the saturation voltage Vsat of transistor QA, with Vsat typically being less than 0.1 volts.
Assuming that input In1 drops in magnitude and assuming the input In2 is still inactive, output transistor QA will tend to turn off. In addition, current IC through transistor QC will increase thereby causing the current ID through transistor QD to drop. This will cause the base-emitter voltage of QD to decrease. Since the base voltage of QD is fixed by VB, the base voltage of transistor QB will drop thereby increasing the base-emitter voltage of output transistor QB and causing QB to turn on harder thereby causing Vout to drop towards VEE. Operation with In2 active and In1 inactive is similar.
Although the FIG. 1 output circuit provides many advantages, including simplicity, the circuit does have shortcomings. By way of example, relatively low impedances at the emitters of transistors QC and QD make the transresistance gain of the circuit very low. Further, the voltage gain of the output stage is not the same for increasing and decreasing inputs. For example, when In1 increases, transistor QC is off, with transistor QD acting as a common base amplifier having a high voltage gain. On the other hand, when In1 decreases, the gain is reduced. This difference in voltage gain makes the amplifier output stage more difficult to stabilize.
An amplifier output circuit and method are disclosed. The output circuit includes a level shifting circuit configured to receive a differential input signal having first and second components. Typically, the differential input signal is in the form of a differential current output signal as produced by a differential input stage. The level shifting circuit defines first and second current paths, each having a transistor connected in series with the respective path.
The level shifting circuit is followed by a driver stage having a first input coupled to the first current path of the level shifting circuit and a second input coupled to the second current path of the level shifting circuit. The driver stage produces first and second outputs indicative of the voltage levels on the first and second current paths, respectively.
The output circuit further includes a common mode feedback path configured to alter current flow in the first and second current paths in response to the voltage levels on the first and second current paths. In one embodiment, the common mode feedback path further provides an output current limit function.
The driver stage of the amplifier output circuit drives an output stage. The output stage includes a further transistor having a base coupled to the first output of the driver stage and coupled between a first power supply rail and an amplifier output. In one embodiment, the transistor is a PNP transistor having an emitter coupled to a power supply rail which is positive with respect to a second power supply rail. A still further transistor has a base coupled to the second output of the driver stage and is coupled between the amplifier output and the second power supply rail. In one embodiment, the transistor is a NPN transistor having a collector coupled to the amplifier output and an emitter coupled to a power supply rail which is negative with respect to the first power supply rail.